a geological report of central strath, skye

i

The University of Liverpool

A Geological Report of Central Strath, Skye In partial fulfilment for the requirements for the degree of BSc Geology

Isaac Onyett 12-14-2016

i

DECLARATION

NAME: ISAAC ONYETT STUDENT NUM. 201029021

I hereby certify that this dissertation constitutes my own product, that quotation

marks indicate where the language of others is directly reproduced, and that

appropriate credit is given where I have used the language, ideas, expressions or

writings of another to inform my work.

I declare that the dissertation describes original work that has not previously been

presented for the award of any other degree of any institution.

I confirm that I have read and understood the University of Liverpool’s Code of

Practice on Academic Integrity. I confirm that I have not committed plagiarism when

completing the attached piece of work, nor have I colluded with any other student in

the preparation and production of this work.

SIGNATURE DATE

ii

Abstract

The area of central Strath is very rich in its geology, with sedimentary rocks ranging

in age from Pre-cambrian to Early Jurassic. The sequence stratigraphy of central

Strath is separated by two unconformities, one of which can be attributed to the

Caledonian orogeny. Post-Caledonian stratigraphy is characterised by an overall

marine transgressions, with frequent minor fluctuations in sea level. Pre-Caledonian

formations are segregated further by an additional unconformity, representing a

transition from a broadly terrigenous environment to shallow, carbonate seas.

Evidence for the Caledonian orogeny is present as a sequence of Pre-cambrian

sandstones that have been thrusted on top of Ordovician dolostones. The British

Tertiary Igneous Province is represented as a collection of intrusive igneous rocks,

cross-cutting this stratigraphic sequence. The intrusive igneous rocks of central Strath

comprise a suite of basaltic and rhyolitic dykes, a large composite sill, and the Beinn

an Dubhaich granite. A collection of NW-SE orientated dykes provide evidence for an

extensional stress regime with a NE-SW directed minimum principal stress axis. Dykes

are cross-cut by the Tertiary Beinn an Dubhaich Granite, which is hosted within a

large anticline to the north of the investigated area. Intrusion of the granite has led

to spectacular deformation of Ordovician dolostones, with calc-silicate mineral

assemblages of increasing metamorphic grade emerging as the Beinn an Dubhaich

granite is approached.

Acknowledgements

I would like to thank both of my supervisors, Professor John Wheeler and Dr. Janine

Kavanagh for all the help and support throughout this project. I also owe thanks to

Adeel Ahmed, Jake Bookham, Alix Crane-Russel, and Sarah Kanwar for their company

and plentiful moral support throughout weeks of mapping and research in Scotland’s

relentless weather.

iii

Abstract ........................................................................................................................ ii

Acknowledgements ...................................................................................................... ii

1 Introduction ......................................................................................................... 1

1.1 Mapping Area ................................................................................................ 1

1.2 Project Aims ................................................................................................... 3

1.3 Fieldwork Techniques .................................................................................... 3

1.4 Geological Setting .......................................................................................... 4

2 Stratigraphy of Central Strath .............................................................................. 6

2.1 Summary of Formations and GVS ................................................................. 6

2.2 Lonachan Sandstone Formation .................................................................... 8

2.2.1 Lithological Description .......................................................................... 8

2.2.2 Environment of Deposition .................................................................. 10

2.3 Bheinn Shuardail Dolostone Formation ...................................................... 12

2.3.1 Lithological Description ........................................................................ 13


2.4 Buidhe Conglomerate Formation ................................................................ 17


2.4.1.1 Quadrat Analysis of the Buidhe Conglomerate Formation .......... 18

2.4.1.2 Sedimentary Log ........................................................................... 20


2.5 Broadford Limestone Formation ................................................................. 24


iv


2.6 Starsaich Sandstone Formation ................................................................... 27



3 Igneous Geology ................................................................................................. 31

3.1 Strathiad Dyke Swarm ................................................................................. 31

3.1.1 Dyke Analysis ........................................................................................ 31

3.1.2 Lithological Descriptions ...................................................................... 32

3.1.2.1 Basaltic Dykes ............................................................................... 32

3.1.2.2 Rhyolitic Dykes .............................................................................. 33

3.1.3 Interpretations ..................................................................................... 33

3.2 Beinn an Dubhaich Granite ......................................................................... 33


3.2.2 Interpretation and Cooling History ...................................................... 35

3.3 Beinn nan Carn Composite Sill Complex ..................................................... 36


3.3.1.1 Microgranite ................................................................................. 36

3.3.1.2 Porphyritic Basalt .......................................................................... 37

3.3.2 Interpretations and Emplacement Mechanism ................................... 39

4 Metamorphic Geology ....................................................................................... 41

4.1 Contact Metamorphism of the Bheinn Shuardail Dolostone Formation .... 41

4.1.1 Chert Nodule Analysis .......................................................................... 41

4.1.2 Reaction Pathway ................................................................................. 43

v

4.1.3 Implications for Granite Emplacement ................................................ 46

5 Structural Geology ............................................................................................. 48

5.1 Ductile Deformation .................................................................................... 48

5.1.1 Broadford Anticline .............................................................................. 48

5.1.2 Strathiad Syncline ................................................................................. 49

5.1.3 Small Scale Folds .................................................................................. 49

5.2 Brittle Deformation ..................................................................................... 51

5.2.1 Faulting ................................................................................................. 51

5.2.1.1 Broadford Thrust Fault.................................................................. 51

5.2.1.2 Other Faults .................................................................................. 52

5.2.2 Mineral Veins ....................................................................................... 52

5.3 Structural Setting of Igneous Intrusions ...................................................... 54

5.3.1 Strathiad Dyke Swarm .......................................................................... 54

5.3.2 Beinn an Dubhaich Granite .................................................................. 55

6 Geological History of Central Strath, Skye ......................................................... 57

7 Conclusion .......................................................................................................... 60

8 References .......................................................................................................... 62

9 Appendices ......................................................................................................... 65

-1-

1 Introduction

1.1 Mapping Area

This thesis is a report of 35 days of fieldwork that was undertaken in Strath, Isle of

Skye, Scotland between 3rd July and 23rd August 2016. The Isle of Skye lies off the

western coast of Scotland, separated from the mainland at its closest point by Loch

Alsh. The island is the largest of the Inner Hebrides and is world renowned for its

dramatic landscapes fashioned by its complex and intricate geology, making the Isle

of Skye so appealing to walkers and climbers. The Isle of Skye, meaning ‘cloud island’

owes its name to the famed Cuillin Hills, which are often dramatically cloaked with

mist. Strath is situated to the east of the Cuillin Hills, breaching Elgol and Kilmarie at

the eastern side of Loch Slapin. The investigated area is in Central Strath, and is

confined by the small villages of Broadford, Torrin, and Heaste, spanning

approximately 23km2. The area is easily accessible by car via the B8083, which runs

south from the A87 at Broadford.

-2-

Figure 1: Location maps of the investigated area. a) highlights the location of the Isle of Skye within

Scotland. b) highlights the location of Strath within the Isle of Skye. c) highlights the confines of the

investigated area.

-3-

1.2 Project Aims

The principle aim of this thesis is to present a detailed and thorough geological history

of the area, integrating various aspects of sedimentary, igneous, and metamorphic

geology, and structural relationships between them.

With relation to the sedimentary geology of the investigated area, the aims of this

project are:

1. Characterise and correlate rock units with respect to their composition and

sedimentary structures

2. Analyse palaeoecology using fossil content of the local sedimentary rocks

3. Construct an integrated history of the deposition and evolution of the local

sedimentary rocks

With relation to the igneous and metamorphic geology of the investigated area, the

aims of this project are:

1. Analyse mineral assemblages in and around the Beinn an Dubhaich pluton

2. Quantify pressure and temperature conditions experienced within the aureole of the

pluton

3. Determine magma source and pluton emplacement mechanism

4. Construct an integral history of intrusion, deformation, and metamorphism

1.3 Fieldwork Techniques

In order to achieve the project aims, a number of field techniques were utilised,

including:

Mapping, using outcrop patterns and topographic signatures to place boundaries

between lithologies, both solid and drift.

Detailed sedimentary logging

Quadrat analysis of clast compositions and orientations

Construction of cross-sections

Mineral modal abundance data collection

Stereographic projection

Petrographic descriptions

-4-

1.4 Geological Setting

The oldest rocks of the Isle of Skye, and some of the oldest rocks in Europe, are

Lewisian Gneisses, which outcrop extensively on the Sleat Peninsula. The rocks have

been intensely deformed and unroofed since their deposition approximately 2,800

million years ago. Succeeding a sustained period of uplift and erosion of the Lewisian

Gneisses, the coarse, pebbly sandstones of the Torridonian were laid down by high

energy fluvial channels in a hot and arid climate. At this time Skye was part of a

supercontinent with much of Canada and Greenland (Stephenson and Merritt, 2002).

By the Ordovician period 550 million years ago, the north-west margin of Scotland

sat on the south-east margin of the supercontinent and had been eroded to a

landscape with flat, inferior topography bordered by an extensive continental shelf.

Calm, shallow seas facilitated the deposition of limestones, along with sandstones

and siltstones from the Cambrian to the Middle Ordovician.

There is no geological record on the Isle of Skye for the following 200 million years.

This hiatus is a consequence of the Caledonian Orogeny to the south-east, which saw

the collision of Laurentia and Baltica, forming the Caledonian mountain belt of which

has roots that form the foundations of Scotland.

The Triassic marks the recommencing of the geological record with repeated deposits

of silt, sand, and gravel across broad floodplains in an arid desert environment.

A marine transgression from the south, thought to be related to increased activity at

mid-ocean ridges during the break-up of Pangea lead to a progressive marine

transgression across the Isle of Skye and the UK. As the landscape became submerged

by a warm, shallow, tropical sea teaming with life, deposition of a sequence of

fossiliferous limestones, sandstones, and mudstones was extensive within the

subsiding Hebrides basin, as described by Morton (1990). Towards the end of the

Jurassic, regional drowning of the landscape deposited a thick blanket of mudstone.

The Late Jurassic deposits are succeeded by another hiatus in the geological record

marked by the Late Cimmerian unconformity in which rifting of the North Atlantic

during the break-up of Pangea lead to widespread erosion and volcanic activity across

the UK. Consequently, basaltic lava flows related to the earliest phase of volcanism

in the region make up the majority of northern Skye. These basaltic lava flows are

commonly known as the Skye Main Lava Series (Miller, 2005). The roots of substantial

central volcanoes associated with this North Atlantic rifting are represented on Skye

by the Skye Central Complex, which includes the Cuillin and Red Hills.

-5-

Figure 2: Summary geological map of the Isle of Skye. The area of study is highlighted. After

(Stephenson and Merritt, 2002)

The area of Strath holds a diverse variety of sedimentary, igneous, and metamorphic

rocks, which are expressed by its dramatically heterogeneous landscape. The rock

types that are subject of this study are representative of the diverse geology of Skye,

with sediments ranging in age from the Pre-Cambrian to the Lower Jurassic. These

sediments have been intruded into by mafic and felsic dykes and sills of the Skye

Central Complex, alongside the Beinn an Dubhaich pluton, which has been intensively

studied by Whitten (1961), and King (1960) among others.

-6-

2 Stratigraphy of Central Strath

2.1 Summary of Formations and GVS

The stratigraphy of central Strath consists of five formations ranging in age from Pre-

cambrian to Lower Jurassic. The Lonachan Sandstone Formation and Bheinn

Shuardail Dolostone Formation constitute the pre-Caledonian lithologies of central

Strath. Post-Caledonian lithologies consist of three conformable formations; the

Buidhe Conglomerate formation, Broadford Limestone Formation, and the Starsaich

Sandstone Formation. The pre-Caledonian formations are isolated by an

unconformity. This unconformity represents a 200 Ma hiatus in deposition as a

consequence of uplift and erosion during closure of the Iapetus Ocean. Above this

unconformity are conformable conglomerates, limestones, mudstones, and

sandstones. A second unconformity intervening the Lonachan Sandstone Formation

and Bheinn Shuardail Dolostone Formation segregates the pre-Caledonian formation.

This unconformity represents a hiatus in deposition lasting approximately 500 Ma.

-7-

Figure 3: Generalised vertical section, comprising summarising the chronostratigraphy,

lithostratigraphy, and biostratography of central Strath

-8-

2.2 Lonachan Sandstone Formation

The Lonachan Sandstone Formation outcrops extensively around Bheinn Shuardail and

Bealach a’ Ghlinee. The rock is particularly well exposed along the lower parts of the

north-west facing flank of Bheinn Shuardail and to the south-east of Loch Lonachan.

Exposure is generally moderate to good.

Type locality: Locality 62 GR(163635,822215)

2.2.1 Lithological Description

The minimum thickness of the Lonachan Sandstone Formation is 255 metres,

although the base is not seen. The sandstone is weakly bedded on a decimetre scale.

Some correlation can be observed between grain size and bed thickness, with

thicker beds generally consisting of larger grains. The sandstone displays infrequent

cross bedding and planar laminations with prominent syn-sedimentary structures

throughout. Planar and wavy laminations are seen in finer beds with distinct lamina

of very fine sand to silt sized dark grey minerals. Figure 4 shows convolute

lamination towards the bottom of the image, passing upwards into wavy

laminations, succeeded further by planar laminations towards the top of the image.

Figure 4: Convolute lamination, wavy lamination, and planar lamination within a bed of the

Lonachan sandstone. Assisted by a sketch of convolute lamination (bottom left). Compass clinometer

for scale. Taken at GR(162845,818700)

-9-

Complex folding of heavy-mineral laminae is prevalent throughout the Lonachan

sandstone. Folds are primarily tight and overturned with variation in the complexity

of deformation within decimetre scale beds. In figure 5 the complexity of

deformation of laminae increases upwards from a broad synformal structure to

multiple small, complex folds.

Figure 5: Complex folding of dark mineral lamina in the Lonachan sandstone. Complexity of

deformation increases towards the top of the bed. Assisted with a sketch of intensely deformed area

with inferred water escape pathways. Pencil for scale. Taken at GR(162638,818593)

The Lonachan sandstone has a reddish brown appearance on a fresh surface and

exhibits no major discolouration due to weathering. The Lonachan sandstone

comprises primarily fine to medium (125-500μm) sized grains of quartz (45%), K-

feldspar (20%), plagioclase feldspar (10%), and lithic fragments (5%). The sandstone

is grain supported with 10% silica cement manifested as syntaxial overgrowths that

glimmer in sunlight. Plotting modal abundances of the constituent minerals on the

Pettijohn classification of sandstones, or the ‘Toblerone plot’ (Figure 6) places the

sandstone in the arkosic arenite field.

-10-

Figure 6: 0% matrix sub-

triangle of the Pettijohn

classification of sandstones,

or ‘Toblerone plot’. Lonachan

sandstone composition is

marked as an orange cross

within the arkosic arenite

field.

2.2.2 Environment of Deposition

The Lonachan Sandstone Formation is analogous to the Applecross Formation of

the Torridonian supergroup, which has been dated to the Pre-cambrian using

fossilised worm burrows, and Rb-Sr dating by Moorbath (1969).

The complex morphology of structures within the sandstone, alongside significant

variations in the complexity of deformation within decimetre scale beds, suggests

that they are related to syn-sedimentary processes and are not of tectonic origin.

Structures of this nature are related to soft sediment deformation, in which water-

escape is an intrinsic cause. Upwelling of water from below the deformed region

causes fluidization of the overlying sediment, leading to tight, complex folds, often

with fragmented hinges. A water escape pathway can be inferred by tracing fold

hinges upwards through multiple deformed laminae (Figure 5). Soft sediment

deformation structures within the Applecross Formation have been described

previously by Owen (1995), who invites the possibility that liquefaction is

seismically induced. The scarcity of cross bedding within the formation may be a

consequence of post-depositional deformation of foresets. Where cross bedding

can be identified, foresets dip towards the south-east at 058o, indicative of a south-

east directed palaeocurrent. Figure 5 represents a progressive change in flow

energy from downstream migration of approximately 1cm amplitude bedforms in

the lower flow regime to downstream migration of very low amplitude bedforms in

-11-

the upper flow regime. Development of laminations in the upper flow regime

(upper plane bed) is indicative of a high-velocity flow. Flow velocity can be

quantified using the flow regime diagram (Figure 7), which indicates a mean flow

velocity of 85 – 135 cm/s. The interpretation of a high energy depositional

environment is supported by a lack of fossils in the sandstone.

Figure 7: Flow regime diagram.

Red dashed lines indicate the

mean flow velocity required for

transportation of fine to medium

sand in the upper flow regime

(upper plane bed). After Harms,

Southard, and Walker (1982)

A relatively high modal abundance of unstable feldspar grains within the Lonachan

sandstone indicates that it is compositionally immature. The constituent grains are

sub-rounded and moderately sorted also indicated that the sandstone is texturally

sub-mature. Compositional immaturity and textural sub-maturity suggest a low

degree of transportation from a nearby source to allow unstable feldspar grains to

prevail during sediment transport.

An absence of floodplain deposits such as muds and silts suggests sustained

deposition from multiple fluvial channels during a period of high subsidence rates.

It can thereby be concluded that the depositional setting of the Lonachan Sandstone

is a high energy braided fluvial system with sediment source in close proximity to

the region of sediment deposition.

-12-

Figure 8: A schematic block diagram representing a potential depositional environment of the

Lonachan sandstone in a braided river system.

2.3 Bheinn Shuardail Dolostone Formation

The Bheinn Shuardail Dolostone outcrops extensively towards the top of Bheinn

Shuardail and envelopes the Beinn an Dubhaich Granite at the centre of a broad

antiform. The rock is particularly well exposed around the peaks Bheinn Shuardail and to

the south of Beinn an Dubhaich. The best exposure of this formation is within quarries

peripheral to the granite. The dolostone is characterised by a karstic weathering profile

and rich green vegetation dominated by bracken in the south and dense woodland to the

north. Exposure is generally very good to excellent.

Type Localities: Locality 37 GR(162750, 820295) (dolostone D1)

Locality 38 GR(162175,819150) (dolostone D2)

Locality 48 GR(161290,819060) (dolostone D3)

-13-


The overall thickness of the Bheinn Shuardail Dolostone Formation is approximately

380 metres. The Bheinn Shuardail Dolostone Formation consists of chert-bearing

siliceous dolostones that can be subdivided into three distinct lithologies, D1, D2, and

D3, that appear sequentially towards the Beinn an Dubhaich granite. Crystal size and

texture vary throughout the formation. At distance from the granite, the dolostone

comprises cryptocrystalline micrite. The texture of the rock shows variation to a more

saccharoidal texture with crystals up to 2 mm in diameter towards the periphery of

the granite. Sedimentary structures are absent throughout this formation.

Dolostone D1 is dark blue/grey but appears pale grey on a weathered surface. Crystal

size ranges from micritic to 1 mm where recrystallization is apparent by pale grey

discoloration on a fresh surface. Chert is abundant and manifested as randomly

distributed dark grey, hackly nodules. Although dolostone D1 is fossiliferous, fossils

are small and scarce. Gastropod Hormotoma trentornesis is utilized to date the

formation to the Ordovician.

Figure 9: Photograph of gastropod Hormotoma trentorensis. 5 pence piece for scale. (a) Assisted by an

illustration of Hormotoma trentorensis highlighting main morphological features and their dimensions

(b). Taken at GR(163055,819065)

In addition to Hormotoma trentorensis, dolostone D1 contains small cephalopods

with long axis no greater than 2 cm.

-14-

Figure 10 Photograph of small cephalopods within the Bheinn Shuardail Dolostone. Structure of the

shells has been highlighted. Dashed lines represent inferred structure of the shells 5 pence piece for

scale. Taken at GR(163055,819065)

Dolostone D2 is pale grey on both a fresh and weathered surface. Crystal size ranges

from 1 to 2 mm in diameter. The primary diagnostic feature of this lithology is white

chert that forms nodules and layers. The chert nodules are larger than those in

dolostone D1, with long axes reaching up to 17 cm. Chert nodules tend to form

disconnected layers that run parallel to continuous chert layers.

Figure 11: Photograph of dolostone D2 showing white chert layers and nodules. Compass clinometer

for scale. Taken at GR(162175,819150)

-15-

Recrystallization of chert to euhedral trigonal pyramids of quartz is recurrent towards

the centre of large nodules, a likely consequence of increased temperatures during

emplacement of the Beinn an Dubhaich granite.

Dolostone D3 is contiguous with the Beinn an Dubhaich granite towards the centre

of the large antiform. Its appearance is very pale grey to off white on a fresh surface,

however weathers pale grey. Dolostone beds are defined by thin 0.5 – 2.5 cm layers

of dark grey chert that are commonly folded and boudinaged.

Figure 12: Photograph of dolostone D3 showing folded dark grey chert layers. Axial trace of folds are

highlighted by red dashed lines. Pencil for scale. Taken at GR(161480,820160)


The Bheinn Shuardail Dolostone is representative of a warm, shallow, tropical,

carbonate marine environment, facilitating the precipitation of calcite. These

dolostones are regarded in literature as a subdivision of the ‘Durness Group’

(Nicholas, 1994). Deposition of dolostones on a stable, gently sloping shelf along a

northern passive margin is described by Brück et.al (2011). Deposition must have

been succeeded by a period of diagenetic dolomitization of the antecedent

-16-

limestone, which may be responsible for an absence of sedimentary structures. The

presence chert within this formation may be attributed to recrystallization of

siliceous organisms during diagenesis. Nodules of chert are likely to be a consequence

of recrystallization of macro-organisms such as siliceous sponges and corals. Layers

of chert are likely to be a product of recrystallization of siliceous skeletons of

microorganisms that have settled to the bottom of the water column, forming a layer

of a siliceous ooze. The transition from chert nodules to chert layers towards the

centre of the antiform may be representative of a regressive system. Where sea level

is relatively high, the primary source of silica is pelagic fallout of siliceous

microorganisms forming continuous layers of chert. Nodules of chert may

characterise a shallower marine environment in which benthic siliceous macro

organisms are the salient source of silica and pelagic fallout of microorganisms in

negligible.

Figure 13: Schematic block diagram representing a potential depositional environment of the Bhainn

Shuardail Dolostone in a shallow carbonate marine environment. Biological causes of chert nodules

and layers are annotated.

-17-

Subsequent dolomitisation of this formation is described by Swett (1969). The

occurrence of fossils within this formation, including Hotmotoma trentorensis has

allowed biostratigraphical dating to the Ordovician.

2.4 Buidhe Conglomerate Formation

The relatively small thickness of the Buidhe Conglomerate is reflected by its outcrop

style as a narrow strip orientated to the north-west from Bheinn nan Carn, extending

towards Broadford. The Buidhe conglomerate also outcrops locally to the north-west

of Bheinn Shuardail near the B8083. The rock is particularly well exposed to the north

of Bheinn nan Carn and east of Loch an Ellein. Exposure is generally moderate to

good.

Type Localities : Locality 67 GR(164855,821810) (coarse sand matrix)

Locality 69 GR(163805,819350) (red mudstone matrix)

Locality 73 GR(163470,818830) (micrite matrix)


The overall thickness of the Buidhe Conglomerate Formation is approximately 90

metres. The Buidhe Conglomerate Formation consists of polymictic

paraconglomerates, which due to sporadic and localised variations in composition

are mapped as one unit. Structures within the Buidhe conglomerate are rare, with

one set of cross bedding within a predominantly sandy bed, and infrequent graded

bedding. Bedding planes were often difficult to establish, defined by abrupt changes

in grain size.

The composition of the matrix varies throughout the formation between coarse sand,

red mudstone, and micrite. Although there is some degree of local variation in matrix

the percentage of matrix by volume can also be seen throughout the formation, from

20% to 85% matrix. The dominant composition changes from coarse sand, to red

mudstone, to micrite from the north-east to the south-west respectively.

-18-

Figure 14: Photograph of the Buidhe Conglomerate. A conglomerate with a red mudstone matrix is

overlain by conglomerate with a micrite matrix. Geological hammer for scale.

2.4.1.1 Quadrat Analysis of the Buidhe Conglomerate Formation

Analysis of clasts within 0.5m2 quadrats was conducted at six localities within the

Buidhe conglomerate to acquire an indication of flow energy and the depositional

environment in which this formation was formed.

A rose diagram plot comprising orientations of clast in all 6 quadrats reveals a

relatively strong preferred orientation to the north-east/south-west. This is

suggestive of a palaeocurrent flowing to the north-east or south-west.

Figure 15: A rose diagram displaying

orientations of clasts from six quadrats across

the Buidhe conglomerate. A relatively strong

NW/SE trend can be seen.

-19-

A clast size frequency histogram compiling all quadrat data (Figure 16) shows an

exponential decrease in clast size against frequency. Although the majority of clasts

are below 3cm, there is a significant number of large clasts, with clasts up to 15.2cm

in size. A large spread of clast sizes indicates very poor sorting of clasts in the

conglomerate. A flow with immense energy is required for the transportation and

subsequent deposition of such clasts.

Figure 16: Clast size frequency histogram displaying the relationship between clast size and frequency

within the Buidhe conglomerate. There is an exponential decrease in clast frequency with increasing

size. However, there is a considerable number of large clasts within the formation.

Figure 17 displays the variation in composition and maximum clast size throughout

the conglomerate, from the north-east to the south-west. The amount of quartz in

the conglomerate appears to decrease from the north-east to the south west, with

dolostone (BSD) being the most abundant clast in the south-west. As well as the

Bheinn Shuardail dolostone, intraformational clasts of the Lonachan sandstone are

also present at (162730,821855).

Maximum clast size is significantly higher is conglomerates with micrite (Lst) and

sandstone (Sst) matrix. This is suggestive of a calmer depositional system during

deposition of the conglomerate with a red mudstone (Mud) matrix. A high flow

0

10

20

30

40

50

60

70

80

90

100

1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6 - 7 7 - 8 8 - 9 9 - 10 >10

FREQ

UEN

CY

CLAST SIZE (CM)

Conglomerate Clast Size Frequency Histogram

-20-

energy must have been sustained througout sediment transport as there is no

progressive decrease in clast size indicative of decreasing flow energy.

Figure 17: Stacked bar chart showing maximum clast size and composition across the Buidhe

conglomerate from the north-ease to the south-west. Composition is based on number of clasts, not

percentage by volume.

2.4.1.2 Sedimentary Log

A 10.8 metre section of the lowermost fraction of the Buidhe Conglomerate

Formation was logged to reveal relationships between compositional variation

within the formation and changes in environment facies.

The base of the section consists of sandstones that do not correspond to other

lithologies within the studied area. Sandstone passes up into red mudstone and

siltstone, succeeded by repeated variations between conglomerates with a coarse

sand matrix and conglomerates with a micrite matrix.

2

4

6

8

12

14

16

164075,819550 163805,81935

GRID REFERENCE

163445,818605 162410,81845

10

CLA

ST S

IZE

(CM

)

0 164855,82182 162730,821855

NE SW

Quartz

Metaquartzite

Dolostone (BSD)

Sandstone (LS) Metapelite

Chert

Sst Sst Sst Mud Lst Lst

-21-

Figure 18: Sedimentary log of a 10.8 metre section of the Buidhe conglomerate and unidentified

sandstones. Cyclic changes in environment are represented here by repeated variation in composition

of conglomerates. Log locality: GR(164320,819650)


The Buidhe Conglomerate Formation is equivalent to the Stornoway Formation, as

described in the literature (Steel & Wilson, 1975). Although a lack of fossil content

makes dating difficult, palaeomagnetic evidence has been utilized by Storetvedt and

-22-

Steel (1993) to date the base of this formation to the Late Permian - Triassic.

Storetvedt and Steel assign a thickness of 4 km to the formation, thus Late Permian

– Triassic stands as an age limit for the conglomerate. Intraformational clasts of the

Lonachan Sandstone Formation and Bheinn Shuardail Dolostone Formation within

the conglomerate represent an unconformity in which the preceding lithologies have

been uplifted, exposed at the surface, and eroded.

The textural immaturity of the Buidhe conglomerate is indicative of a minor degree

of transportation from a proximal source. Flow energy must have been sufficient to

transport clasts up to 15.2cm in size. Minimum flow speed can be quantified using

the Hjulstrom curve (Figure 19), which proposes a high-velocity flow of 110 cm/s.

Figure 19: Hjulstrom curve highlighting the minimum flow speed required to transport clasts 15.2 cm

in size within the Buidhe conglomerate. A high-velocity flow speed of 110 cm/s is required.

Deposition of the conglomerate may have been within an estuarine environment on

a continental plain, accompanied by fluctuations in sea level. Although not present in

the investigated area, the widespread calcretes within the formation suggests an arid

climate during deposition (Morton and Hudson, 1995). The conglomerate with a

coarse sand matrix may have been deposited by a fluvial channel, possibly as crevasse

splay deposits where flow energy peaks at the outermost margin of a meander.

Where the fluvial channel meets the less confined shallow marine, flow energy

-23-

decreases and clasts will be deposited instantaneously. Background precipitation of

calcium carbonate in the shallow marine encases the clasts in a fine micrite matrix. A

red mudstone matrix is indicative of a calm subaerial environment with incidental

background settling of fine particles. Conglomerates with a red mudstone matrix may

have been deposited in floodplain lakes.

A progressive change in matrix composition from coarse sand in the north-east to

micrite in the south-west with intervening conglomerates with a red mudstone

matrix may represent different sub-environments within an estuary. This is

suggestive of fluvial channels dominating the north- east, feeding sediment to a

carbonate sea to the south-west. Potential sub-environments responsible for

variations in matrix composition are highlighted in figure 20.

Figure 20: Block diagram representing a possible depositional environment for the Buidhe

conglomerate, with sub-environments responsible for compositional variation within the conglomerate

highlighted.

Local variations in matrix composition, seen for example in the logged section (Figure

18) may be representative of fluctuations in relative sea level. This will cause

transitions between fluvial-dominated and marine-dominated environments, and

subsequently will be marked as localised changes in matrix composition in the rock

record.

-24-

2.5 Broadford Limestone Formation

Exposure of the Broadford Limestone Formation is generally poor where the

topography is uniform and flat, however the presence of this formation can often be

deduced by the appearance of bracken, which tends to grows on calcareous soils. The

best exposure of this formation can be seen as cliff sections to the north of Beinn nan

Carn, and along river sections, namely Allt na Cloiche Bideiche and Allt a’ Choire.

Type locality: Locality 74 GR(163650, 818805)


The overall thickness of the Broadford Limestone Formation is approximately 310

metres. The Broadford Limestone Formation consists of limestones and sandstones

interbedded on a decimetre scale. Limestone also occurs less frequently as lenses

within sandstone beds.

The limestones are dark grey to dark blue on a fresh surface, however weather pale

grey. The limestones are fossiliferous, dominated by complete bivalve shells that are

often articulated. The majority of limestone beds within the formation are mud

supported, with fossil content exceeding 10% of the rock by volume. Thus, a

biomicritic wackestone is inferred. Bivalve Liostrea birmanica (Figure 21) has been

utilized to date the formation to the Lower Jurassic.

Figure 21: Photograph of bivalve

Liostrea birmanica. Outline of shell

and internal structure have been

highlighted. Taken at

GR(163650,818805)

-25-

The limestone is predominantly arenaceous with a notable transition at the base of

the formation from pure limestone to arenaceous limestone. Quartz grains range in

size from very fine sand to small pebbles (0.3mm to 35mm).

Figure 22: Photograph of a particularly quartz-rich bed of limestone with an abundance of pebble-sized

quartz clasts. Compass clinometer for scale. Taken at GR(165610,821855)

Sandstone beds are typically considerably thinner than limestone beds and tend to

weather out as resistant ribs. They are often undular and sometimes pinch and swell.

The sandstone appears grey on a fresh surface, with buff weathered surfaces. Grains

size ranges from very fine to coarse sand (187 μm – 350 mm), with a well sorted and

sub-rounded texture. The sandstone is grain supported, and is composed exclusively

of quartz. A silica cement is inferred by an inertia when in contact with hydrochloric

acid and high competence. Hummocky cross stratification, although rare, is also

present.

-26-

Figure 23: Photograph of a typical sequence of limestone and sandstone beds of the Broadford

Limestone Formation. Relative sizes of beds are typical here and thin sandstone bed can be seen

undulating towards the top of the image. A5 notebook for scale. Taken at GR(163650,818805)


The Broadford Limestone Formation is analogous to the Breakish Formation,

previously known as the Lower Broadford Beds (Morton et al., 1999). Repeated

alternations between arenaceous biomicritic wackestones and quartz sandstones

may reflect cyclic changes in climate or sea level, which subsequently influences

sediment supply.

Alternatively, beds of quartz sandstone may represent storm events, activating and

reinforcing fluvial channels that subsequently introduce terrigenous sediment into

the system. This notion is supported by the presence of hummocky cross

stratification within sandstone beds, indicative of deposition above storm-weather

wave base. This is in agreement with Simms et al. (2004), who propose that the

sandstone beds represent offshore sandbars, with wave, tide and possibly storm-

action evident. Additionally, large pebble sized clasts within the limestone at

GR(165610,821855) require substantial flow energy for transportation and

deposition from the provenance of the clasts to a carbonate sea. An abundance of

un-fragmented, commonly articulated bivalve shells within limestone beds are

indicative of a calm marine environment alternating with storm events that inject

terrigenous sediment into the system.

-27-

Depositional environment is likely to be a shallow carbonate sea in close proximity to

the mouth of at least one river feeding the system with terrigenous sediment. This

hypothesis is supported by the presence of Liostrea, which are characteristic of very

shallow water marginal marine environments (Hallam, 2009). Possible depositional

settings are highlighted on figure 24.

Figure 24: Schematic block diagram representing a potential depositional environment of the

Broadford limestone in a shallow carbonate marine above storm-weather wave base. Storm events

cause an influx of terrigenous sediment through reinforcement of fluvial channels.

2.6 Starsaich Sandstone Formation

The Starsaich Sandstone lies conformably above the Broadford Limestone, and also

outcrops in low-lying regions with inferior topography. This formation outcrops most

extensively at the centre of a large synform to the east of the investigated area.

Exposure is generally poor due to heavy vegetation, so sections cut by rivers are

utilized in the study of this formation.

Type locality: Locality 18 GR(163320,822380)

-28-


The minimum overall thickness of the Starsiach Sandstone Formation is

approximately 180 metres, although the top of this formation is not seen. The

Starsaich sandstone comprises sandstones and micaceous mudstones and siltstones

interbedded on a decimetre scale.

The sandstone has a light to dark grey appearance on a fresh surface, however

weathers brown. Sandstone beds display infrequent tabular and hummocky cross

stratification. The constituent grains are primarily medium (250-500 μm) in size and

are dominated by quartz (85%), with some plagioclase feldspar (5%) and lithic

fragments (10%). Muscovite mica constitutes approximately 5% of the sandstone

and is persistent throughout the formation. Plotting modal abundances of the

constituent minerals on the Pettijohn classification of sandstones, the ‘Toblerone

plot’, reveals that its composition is that of a sublitharenite. Effervescence with

dilute hydrochloric acid signifies that grains are secured by a carbonate cement.

Mudstones and siltstones are dark grey to black on both weathered and fresh

surfaces. Laminations are common, with degree of lamination often corresponding

to grain size. Mudstone beds tend to have more prominent laminations and a higher

degree of fissility. Gain size ranges from clay (< 2 μm) to silt (2-63μm). Mudstone and

siltstone beds are relatively rich in muscovite mica (10%), which is prevalent

throughout the formation. A large proportion of mudstone and siltstone beds are

fossiliferous, dominated by bivalve Gryphaea arcuata.

Figure 25: Photograph of a fossilized Gryphaea arcuata (a) assisted by an illustration highlighting some

morphological features of the shell (b). Taken at GR(165680,821390)

-29-

Casts of Gryphaea are common, as well as shells that have been replaced by calcite,

with rare preservation of shell structure. Orientations of 31 Gryphaea shells within a

1m2 quadrat on a Gryphaea bed reveals a moderate preferred orientation of shells to

the east.

Figure 26: Contoured stereonet displaying

orientations of poles of 31 Gryphaea

shells. A high concentration of Gryphaea

shells can be see orientated to the east.

The uppermost subsection of this formation is characterised by a large (25m) unit of

mudstone outcropping towards the centre of the Strathiad synform to the east at

GR(164660,818540).


The Starsaich Sandstone Formation is referred to in the literature as the Ardnish

Formation, previously known as the Upper Broadford Beds (Hesselbo and Coe 1989).

Interbedded sandstones with mudstones and siltstones are representative of cyclic

changes in depositional setting, possibly associated with fluctuations in climate, sea

level, or a combination of the two. Hummocky cross stratification within sandstone

beds indicates depth of deposition above storm-weather wave base. Contrastingly,

fine laminations within the mudstone suggest settling of fine sediment to substantial

depths within the water column. By the Early Jurassic, Ordovician dolostones (Bheinn

Shuardail Dolostone Formation for the purposes of this project) were completely

submerged underwater (Farris, Oates and Torrens, 1999). This denotes a marine

transgression from the Triassic in which the Ordovician dolostones were subaerially

exposed. Dark grey to black colouration is also indicative of a deep anoxic marine,

-30-

facilitating the preservation of fossils. This marine transgression is described by a

change in palaeoecology from shallow water Liostrea, to Gryphaea, typical of deeper,

more open water environments in which ammonites also occur (Hallam, 2009).

A relatively high modal abundance of quartz accompanied by low a proportion of

unstable feldspar grains suggests that the sandstone is compositionally mature. A

well-sorted texture and sub-rounded grains is also representative of the textural

maturity of the sandstone. This is indicative of a high degree of transportation of

clasts from their source. Thus, the Starsaich sandstone is inferred to have been

deposited in a distal, relatively deep marine environment with temporal variations in

water depth.

-31-

3 Igneous Geology

3.1 Strathiad Dyke Swarm

3.1.1 Dyke Analysis

Detailed analysis of the Strathiad Dyke Swarm was performed throughout the

project, recording composition, width, strike, and dip of 37 dykes. Extensive

investigation shows that the dyke swarm is dominantly basaltic, with scarce rhyolitic

dykes. Regardless of their composition, all dykes have been inferred to be members

of the same dyke swarm as they are similarly orientated to the north-west/south-

east (see section 5).

Figure 27: Pie chart displaying the relative abundance of dykes of different compositions in central

Strath

-32-

3.1.2 Lithological Descriptions

3.1.2.1 Basaltic Dykes

Basaltic dykes are the most abundant in the investigated area and can be sorted into

aphyric (49%) and porphyritic (41%).

Porphyritic basalt dykes consist of large, milky white, lath-shaped plagioclase

phenocrysts within a very fine, dark grey/green ground mass. Mineralogy of the

ground mass is often hard to determine due to very fine crystal size, however

amphiboles and plagioclase have been inferred from flecks of milky white plagioclase

(10%) among a predominantly dark green/grey amphibole-rich ground mass (65%).

Figure 28: Photograph of a porphyritic basalt dyke. Note that this dyke is particularly rich in plagioclase.

10 pence piece for scale.

Towards the periphery of the Beinn an Dubhaich granite, alteration to metabasites is

inferred from the presence of acicular, sometimes radial and fibrous actinolite

crystals within the ground mass (GR[161630,819950]).

-33-

3.1.2.2 Rhyolitic Dykes

Rhyolite dykes are dominantly porphyritic with phenocrysts of k-feldspar in a fine

ground mass of plagioclase feldspar and quartz.

One amygdaloidal rhyolite dyke outcrops at GR(164865,821790), with large vesicles

(3.5 - 5.0mm) constituting ~80% of the dyke. The vesicles are infilled with plagioclase

feldspar, and inter-vesicle spaces are composed of quartz (~20%).

3.1.3 Interpretations

The presence of phenocrysts within approximately half of the basaltic dykes is

indicative of a period in which the magma resides, fostering large, euhedral

plagioclase crystals during its ascent to the surface. A succeeding period of

crystallisation at shallower depths and lower temperatures encases plagioclase

phenocrysts in a very finely crystalline ground mass.

3.2 Beinn an Dubhaich Granite

The Beinn an Dubhaich granite outcrops within the centre of the Broadford anticline

at the north-east of the investigated area. Exposure is generally very good, with

excellent exposure at highest topographic heights of Beinn an Dubhaich, and along

river sections. The extent of the granite at the surface is clearly outlined by a

punctuated change in vegetation from rich green bracken on the enveloping

dolostone, to deep purple heather on the granite. The impermeability of the granite

is reflected by overlying marshland.

Type locality Locality 26 GR(161110,820345)

-34-


The Beinn an Dubhaich granite is a predominantly coarsely crystalline, equigranular,

felsic igneous rock. The granite consists primarily of anhedral quartz, subhedral to

euhedral k-feldspar, and subhedral plagioclase feldspar with minor amounts of

anhedral biotite mica and hornblende. Peterological investigation along two

transects, spanning both the width (T1) and length (T2) of the pluton has led to its

identification as a mozono-granite, with some local variation to a grano-diorite.

Figure 29: Stacked bar chart showing average modal abundances of the constituent minerals of the

Beinn an Dubhaich granite.

Significant variation in crystal size is apparent along transect T2, which spans the

width of the granite. The general trend shows an increase in average crystal size

towards the centre of the pluton, with maximum crystal size of 2.2 mm in the central

portion. Crystal size is approximately 1.6-1.7 mm towards the contact with the Beinn

Shuardail Dolostone Formation.

Figure 31: Graph showing the variation in average crystal size across the width of the Beinn an

Dubhaich granite (transect T1)

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 100 200 300 400 500 600 700 800 900 1000 1100

Cry

stal

siz

e (m

m)

Distance along transect (m)

Average Crystal Size vs Distance Along Transect

-35-

Figure 31: Graph showing the variation in

modal abundance of the constituent

minerals of the Beinn an Dubhaich granite

using data from transect T2.

The overall composition of the granite is relatively uniform throughout, however

noteworthy vertical stratification of the pluton is evident. A pronounced increase in

quartz and ferromagnesian minerals is seen from the lowermost regions of the

exposed granite towards the highest topographic heights. K-feldspar remains

relatively constant throughout, however the amount of plagioclase increases subtly

with height up the pluton. An increase in modal abundance of quartz with height up

the Beinn an Dubhaich is also documented by Whitten (1961).

3.2.2 Interpretation and Cooling History

Variation in crystal size within the pluton can be attributed to higher rates of

nucleation and slow rates of crystal growth at the contacts with the cold country rock.

-36-

Slower rates of nucleation and high rates of crystal growth in the central portions of

the granite form larger crystals up to 2.2 mm in size.

Vertical stratification within the Beinn an Dubhaich granite may be due to crystal

fractionation throughout cooling. In this process, dense, mafic, ferromagnesian

minerals crystallise out of the melt during early stages of cooling and begin to sink

under gravity. The melt composition becomes progressively more evolved and

continues to cool, causing the upper portion of the magma chamber to be more felsic,

reflected by increased amounts of quartz.

3.3 Beinn nan Carn Composite Sill Complex

The Beinn nan Carn microgranite outcrops most extensively at the highest

topographic heights of Beinn nan Carn, running north-west as a series of hills and

ridges. Exposure is generally very good with best exposures elevated at heights of

approximately 250 metres above sea level.

Type Locality: Locality 57 (GR163505,818380)


3.3.1.1 Microgranite

The Beinn nan Carn microgranite appears light grey on a fresh surface, and shows

no major discolouration due to weathering. The bulk of the microgranite is

structureless, with occasional planar features that resemble foresets.

The felsic composition of this rock unit is reflected by a pale colour index of 5 – 10%.

Crystals are predominantly equigranular medium sized (~1 mm). The mineralogy is

dominated by pale, felsic crystals of quartz (10-15%), K-feldspar (10-20%), and

plagioclase feldspar (12-35%). Dark green to grey amphiboles constitute the scarce

mafic minerals within the rock.

-37-

Figure 32: Photograph of microgranite with thin, planar features that resemble foresets, dipping to the

west. Geological hammer for scale. Taken at GR(163505,818380)

3.3.1.2 Porphyritic Basalt

The microgranite is bound at either side by abrupt contacts with a notably

contrasting porphyritic basalt. The basalt is dark grey on a fresh surface, however

weathers a rust brown, indicative of significant iron content. The mafic mineral

content is significantly higher than the microgranite, reflected by a colour index of

70 – 75%. Plagioclase feldspar constitutes approximately 7- 15% of the ground

mass, however it is also present as large, often euhedral, lath-shaped phenocrysts

constituting 10 -15% of the composition. The remaining composition is dominated

by small dark grey/green amphiboles (70%) with trace amounts (2%) of platy biotite

mica.

The upper-most unit of porphyritic basalt at GR(163500,818450) is cross-cut by a

series of light grey, porphyritic rhyolite veins. The composition of the veins are

comparable to that of the microgranite, however consist of pale pink phenocrysts of

K-feldspar. The ground mass is aphanitic and is inferred to be felsic by a low colour

index of 1 - 5%. Large, white metaquartzite enclaves are also encased in the

porphyritic basalt here. Rhyolite veins cross-cut the enclaves, enabling age

relationships to be established.

-38-

Figure 33: Photograph of a metaquartzite enclave within the porphyritic basalt. Enclave is cross-cut by

a vein of porphyritic basalt. Compass clinometer for scale. Taken at GR(163500,818450)

In addition to veins, enclaves of porphyritic rhyolite are also observed in the upper unit of porphyritic

basalt. Enclaves are small and rounded, with long axis up to 3.5 cm. A preferred orientation of the

blebs to the SSE is apparent at a bearing of approximately 150°.

Figure 34: Photograph of porphyritic rhyolite enclaves within the porphyritic basalt. Rounded form and

NE-SE orientation can be seen. Compass clinometer for scale. Taken at GR(163045,818490)

-39-

3.3.2 Interpretations and Emplacement Mechanism

Although only brief direct contact between the microgranite and porphyritic basalt

is evidenced at GR(164960,819030), the grouping of these two lithologies as one

complex is suggested by the following observations. Firstly, the microgranite and

porphyritic basalt are always adjacent to one another, showing close relationships

in outcrop pattern. Secondly, where the porphyritic basalt is exposed, it is

frequently cross-cut by porphyritic rhyolite veins and hosts rounded felsic enclaves.

The rounded appearance of the enclaves is indicative of a cognate origin, as

opposed to a xenolithic origin if angular (Bédard, 1993) and is suggestive of similar a

timing of emplacement of the two units. Finally, and perhaps most importantly, the

baked margins at the upper and lower contacts between the porphyritic basalt and

country rock are of remarkable size. The upper baked margin of quartzite is 64 cm

thick (Figure 35), and the lower baked margin of mudstone is 45 cm thick at

GR(164645,818705). Metamorphism of this degree requires a sizeable causative

body of magma, which is unlikely to be accounted for by the basalt alone.

Figure 35: Photograph of the contact between the Beinn nan Carn Composite Sill and overlying

sandstones. Magnified image shows Large 64 cm baked margin. Taken at GR(163500,818450)

Cross-cutting relationships between rhyolite veins, enclaves, and porphyritic basalt

(Figure 33) allow a general emplacement history to be determined. As can be seen in

-40-

figure 33, the initial injection of magma would have been of mafic composition,

forming porphyritic basalt. Stoping of the country rock during intrusion has

incorporated sandstone enclaves into the melt, which consequently bakes them to a

metaquartzite. After some degree of lithification of the porphyritic basalt, a

secondary injection of significantly more evolved melt has intruded into the centre

of the partially molten porphyritic basalt. Offshoots of felsic magma intrude into the

adjacent porphyritic basalt, cooling rapidly to form rhyolite veins. This cooling history

proposes that the porphyritic basalt and microgranite are part of the same igneous

complex, as a composite sill (Figure 36). Planar features in the microgranite are

inferred to be a product of weathering of less resistant minerals that have segregated

by friction and viscosity during magma transport to create flow banding. This is

suggestive of palaeoflow direction of magma to the west.

Figure 36: Composite sill model for the Beinn nan Carn Composite Sill Complex

-41-

4 Metamorphic Geology

4.1 Contact Metamorphism of the Bheinn Shuardail

Dolostone Formation

The Bheinn Shuardail Dolostone Formation is the only formation within the studied

area to have undergone significant metamorphism. As expected, the metamorphic

grade increases towards the Beinn an Dubhaich granite. A geological map showing

the distribution of metamorphic zones established using index minerals within

concentrically zoned reaction rims around chert nodules can be accessed at the end

of this chapter.

4.1.1 Chert Nodule Analysis

The first mineral to be seen as a reaction rim enveloping chert nodules as the

granite is approached is talc. Talc is characterised by its white colour, greasy lustre,

and ability to be scratched with a fingernail due to its softness. It forms thin, 1 – 2

mm rims around chert nodules, as seen in figure 37.

Figure 37: Photograph of three chert nodules, each with a thin 1-2 mm reaction rim of talc. 1 pence

piece for scale. Taken at GR(163160,819990)

-42-

The next index mineral to appear as the granite is approached is tremolite.

Tremolite is white to grey in colour, opaque and has a vitreous lustre. A diagnostic

feature utilized to identify tremolite is its bladed or acicular crystal habit (figure?).

Tremolite is the most common index mineral within the reaction rims, with

particular abundance in dolostone D2.

Figure 38: Photograph of radial, bladed crystals of tremolite within a reaction rim surrounding a white

chert nodule in dolostone 2D. Taken at GR(162175,819150)

The highest grade index mineral, thus the final mineral to appear as the granite is

approached is serpentine. Serpentine is yellowish green in colour, conchoidal,

vitreous, and has a hardness greater than 7, reflected by its inability to be scratched

with steel. Where serpentine is present, there is often an absence of chert at the

core of the reaction rims. It is important to note that diopside can also be seen

within reaction rims local to the granite, however diopside is only seen co-existing

with serpentine, and is never the highest grade mineral in any reaction rim.

-43-

Figure 39: Photograph of a thin rim of serpentine fringing a zone of acicular tremolite crystals at the

periphery of a reaction rim. Taken at GR(162170,820140)

4.1.2 Reaction Pathway

In order to infer a reaction pathway for the progressive metamorphism of the

Bheinn Shuardail Dolostone Formation, the behaviour of the system must first be

established. An open system behaviour has been inferred by the presence of

symmetrical metasomatic alteration about joints within the dolostone. Veins of

coarse K-feldspar, inferred to have been sourced from granitic fluids, also supports

an open system. This is suggestive of free movement of fluids in and out of the

dolostones as metamorphism occursand allows the fluid composition to be

externally buffered, and the reaction pathway to appear vertical on a T-XCO2 diagram

(Figure 40).

-44-

Figure 40: T-XCO2 diagram for siliceous dolostones at low pressures (P = 0.1 GPa). Key reaction lines

have been highlighted. Blue shaded area represents the highest grade zone established by chert nodule

analysis. (After Winter, 2001)

The first, and lowest temperature reaction to occur is the talc-in isograd. This is a

cross-tie line reaction in which the Dol-Qtz reaction line is replaced by the Dol-Tlc

reaction line. This is of importance as it implies that the composition of the fluid has

a molal fraction of CO2 below 0.7. An open system with XCO2 above 0.7 would see

tremolite as the lowest temperature index mineral.

Figure 41: Compatibility diagrams for siliceous dolostones (shaded half), showing the introduction of

talc through reaction 1. (After Winter, 2001)

-45-

As temperatures increase towards the granite, tremolite is introduced into the

system through reaction 2a. This is an enclosure reaction in which tremolite appears

within the SiO2, Ca, Tlc sub-triangle.

Figure 42: Compatibility diagrams showing the introduction of tremolite into most siliceous dolostones

(shaded half) through reaction 2b. (After Winter, 2001)

When all the quartz is consumed, the assemblage becomes univariant and

temperature proceeds to rise. At this stage, talc will begin to be consumed in the

cross tie-line reaction 2b. This reaction introduces tremolite into most siliceous

dolostones as the bulk of dolostone compositions lies within Cal-Tr-Dol sub-triangle

(Winter, 2001).

Figure 43: Compatibility diagrams showing the introduction of tremolite to most siliceous dolostones

(shaded half) through reaction 3. (After Winter, 2001)

-46-

Forsterite, is introduced into the reaction rims by reaction 3. In this cross tie-line

reaction the Tr-Dol and Fo-Dol tie lines are replaced by a Cal-Fo tie line.

Figure 44: Compatibility diagrams for siliceous dolostones (shaded half), showing the introduction of

forsterite through reaction 3. (After Winter, 2001)

Serpentine is a product of replacement of forsterite in the presence of hydrothermal

fluids during retrograde metamorphism. Although forsterite is no longer visible in the

dolostones, the point at which serpentine is first observed has been classified as the

‘forsterite in’ isograd.

4.1.3 Implications for Granite Emplacement

Although lithostatic pressure at the time of intrusion of the Beinn an Dubhaich granite

is estimated to be on the order of around 500 bars Hoersch (1979), the pathway

described in figure 40 still applies to low pressure, high temperature metamorphism.

It is important to note that there will be some margin of error in interpretations as

these reactions are pressure dependent. The highest grade reaction line to be crossed

within this system is the Tr + Cal = Fo + Dol reaction line. As a fluid composition with

XCO2 < 7 has already been established, this allows an area within the T-XCO2 diagram

to be constrained, and thus peak temperatures during granite emplacement can be

determined as 460 to 620oC. This concurs with Hoersch (1981) who approximates the

maximum temperatures during emplacement to be 600°C.

-47-

Figure 45: Geological map of the Beinn an Dubhaich area showing distribution of metamorphic zones

and isograds within the Bheinn Shuardail Dolostone

-48-

5 Structural Geology

5.1 Ductile Deformation

5.1.1 Broadford Anticline

The Broadford anitcline trends NE-SW, reflected by repeating stratigraphical units

to the NE and SW. The occurence of way-up structures, such as cross bedding and

graded bedding throughout the studied area suggests that the stratigraphic

sequence is the correct way up, thus an anticlinal antiform is inferred. The hinge

line of the fold is displayed on figure 46a, showing a gentle plunge to the north-east

at 04/044. The fold hinge is arcuate, curving the west towards the north-east. An

axial plane dipping steeply to the south-east at 20/388 is shown on stereonet figure

46a by a red dashed line. A south-westerly dipping axial plane is indicative of a

subtle vergence to the north-west. The geometry of the fold is open, shown by a

high concentration of poles to bedding planes about the best-fit girldle on

contoured stereonet figure 46b.

Figure 46: Stereographic projections showing a) hinge line and axial plane orientation, and b)

concentration of poles to planes about best fit girdle for the Broadford anticline

a) b)

-49-

5.1.2 Strathiad Syncline

The Strathiad syncline is orientated similarly to the Boradford anticline, with a

north-east trending hinge line. As the stratigraphy of central Strath has been

inferred to be the correct way up, a synclinal synform is deduced. An arcuate hinge

line also mimics that of the Broadford anticline. The fold hinge plunges gently to the

south-west at 08/204, shown on stereonet figure 47a. The fold has an open to

gentle geometry, with beds on both limbs dipping more gently than those of the

Broadford anticline. This is reflected by a remarkably high concentration of poles to

bedding planes about the best fit-girdle on contoured stereonet figure 47b.

Figure 47: Stereographic proctions showing a) and axial plane orientation, and b) concentration of

poles to planes about best fit girdle for the Strathiad syncline

5.1.3 Small Scale Folds

Small scale folds are limited to the Bheinn Shaurdial Dolostone Formation, and are

expressed as folded chert layers. Folds are predominantly close to tight, with fold

hinges orientated to the north-west/south-east. A common characteristic of the

small-scale folds an asymmetrical, overturned geometry with vergence to the

a) b)

-50-

south-east. This is suggestive of an uneven distribution of stress during deformation

of the chert layers, with greater amounts of stress from the north-east. Although

uncommon, folded chert layers also appear to be boudinaged. Fragmentation of the

limbs of folds is a consequence of an extensional regime post-dating the period of

folding.

Figure 48: Photograph of a folded and boudinaged chert layer within dolostone D3, assisted by an

illustration highlighting key features. Taken at GR(161290,819060)

Although not always possible, fold hinge orientations are plotted on stereonet

figure 49. When comparing the hinge line orientations of the small-scale folds in

chert layers to the hinge orientation of the Broadford anticline, it is evident that

these folds are not associated with the Broadford anticline as parasitic folds. A best-

fit girdle for the hinge lines orientations within folded chert layers shows a general

orientation of hinge lines to the NW/SE. This is almost perpendicular to the hinge

line of the Broadford anticline, suggesting that they are product of a contrasting

stress regimes. As illustrated in figure 49, the Broadford anticline is inferred to have

formed in a NW/SE directed compressive stress regime, whilst the small scale folds

were formed in a NE/SW directed compressive stress regime. Hinge lines plunging

away from the axial plane of the Broadford anticline suggests that the chert layers

were folded in a preceding compressive episode.

-51-

Figure 49: Stereographic projection comparing hinge line and axial plane orientations of the Broadford

anticline, and hinge line orientations of small-scale folded chert layers. Annotated with inferred

directions of maximum principal stresses.

5.2 Brittle Deformation

5.2.1 Faulting

5.2.1.1 Broadford Thrust Fault

The contact between the Bheinn Shuardail Dolostone Formation and Lonachan

Sandstone Formation can be clearly traced by the use of vegetation (bracken) and

karstic sink-holes in the dolostone. This contact is a thrust sheet, referred to in

literature as the Kilhorn thrust (Bailey, 1954). The surface expression of the thrust

follows the outcrop pattern of beds folded in the Broadford anticline and indicates

that thrusting pre-dates the folding event. This hypothesis is supported by the

displacement of the thrust being exclusive to the pre-Caledonian formations,

suggesting that it is a denouement of the Caledonian orogeny. Intraformational

clasts of Lonachan Sandstone within the Triassic Buidhe Conglomerate also suggests

subaerial exposure of this formation by this time, for which thrusting is likely to be

responsible.

-52-

5.2.1.2 Other Faults

Although no faults were identifiable by the use of displacement markers in the field,

seven faults are inferred by punctuated contacts with juxtaposing lithologies.

Lengths of faults at the surface range from 59 to 301 metres. Down-throw blocks

have been inferred using the rule that the outcrop pattern of the up-thrown block

will shift in the direction of dip. Approximations of fault orientations are plotted as a

rose (Figure 50), which displays their orientation to the NW-SE. This corresponds to

the dominant orientation of dykes in the area which may have formed in response

to the same tectonic event. Displacement by NW/SE trending faults can be seen in

all lithologies, from Pre-cambrian to Lower Jurassic, thus must have formed in a

post-Jurassic tectonic event.

Figure 50: Rose diagram displaying orientations of faults across central Strath. A strong NW/SE trend

can be seen.

5.2.2 Mineral Veins

Quartz veins are abundant throughout the Lonachan Sandstone Formation and are

arranged in a conjugate pattern with dihedral angles of approximately 60°. Quartz

veins have an average thickness of 2 mm, with some veins possessing lengths as

small as 4 cm, and others being more persistent throughout the beds with lengths

up to 79 cm. A dominant vein orientation is not ascertainable due to multiple sets

with differing orientations. However, conjugate joints are orientated to the NW/SE,

-53-

which allows the stress regime at the time of formation to be estimated (Figure 51).

An extensional regime with minimum principal stress direction from the north-east

and south-west is inferred.

Figure 51: Photograph of conjugate quartz veins in the Lonachan Sandstone Formation. Assisted by

a sketch of conjugate veins with inferred principal stress orientations. Tape measure for scale. Taken

at GR(163635,822215.

When vein orientations are expressed as planes on a stereonet, the orientations of

the principal stress axes can be defined as; σ1 = 76/261°, σ2 = 05/151°, σ3 = 13/060°.

Figure 52: Stereographic projection showing the orientations of the principle stress axis forming

conjugate veins, with sense of shear.

-54-

The only other formation that hosts an abundance of mineral veins is the Broadford

Limestone Formation. Veins are composed of calcite and also display a strong NW-

SE trend. The precursory joints would have formed perpendicular to the minimum

principal stress orientation, thus this arrangement of veins would have formed

during an extensional episode with minimum principal stress axis orientated NE-SW.

5.3 Structural Setting of Igneous Intrusions

5.3.1 Strathiad Dyke Swarm

Following detailed analysis of the Strathiad Dyke Swarm, a collection of data including

strike, dip, and width of 37 dykes alongside host rock composition has been collected.

The vast majority of dykes appear within the Bheinn Shuardail Dolostone Formation,

with the least number of dykes appearing in the Lonachan Sandstone. Figure 53

shows that the Bheinn Shuardail Dolostone is the preferential host rock for the dyke

swarm, with clastic sedimentary rocks being less favourable. This may be attributed

to the brittle nature of carbonate rocks facilitating the formation of fractures, which

in turn provide accommodation space for the proceeding magma.

Figure 53: Pie chart displaying the relative number of dykes hosted within each sedimentary lithology

of central Strath

-55-

Regardless of composition, all dykes have a dominantly NW-SE orientation. Although

no cross-cutting relationships are observed, this homogeneous orientation of the

dyke swarm suggests all dykes were emplaced in temporal-proximity. The

emplacement of dykes generally occurs in an orientation perpendicular to the

minimum principal stress axis, thus, the Strathiad dyke swarm was emplaced during

a period in which the minimum principal stress acting upon the rocks had a NE-SW

orientation. This is congruent with the NE-SW extensional stress regime described by

England (1988) in the volcanic episode of the British Tertiary Volcanic Province.

Figure 54: Rose diagram displaying orientations of dykes of the Strathiad dyke swarm. A strong NW/SE

trend can be seen.

Average dyke width is 1.24 meters, with a total cumulative thickness of 45.8 meters.

Although not all of the dykes in the area could be measured, this is a proxy for the

minimum amount of consequential crustal extension in the area.

5.3.2 Beinn an Dubhaich Granite

At GR(161630,819950) the granite can be seen cross-cutting a basaltic dyke of the

Strathiad Dyke Swarm, indicating emplacement in a succeeding magmatic episode.

However, it is likely that intrusion of the granite occurred soon after the dyke swarm

and may be related to the evolution of one initially mafic magma chamber.

-56-

Rafts of the Bheinn Shuardail Dolostone within the main body of the granite appear

to be relatively undeformed, possessing chert layers with strike and dip comparative

to those at the periphery of the granite. It can thus be concluded that the dolostone

rafts are an in-situ product of differential erosion. King (1960) envisaged these rafts

as unroofed members of the pluton floor, proposing a sheet-like intrusion that may

have exploited the thrust fault during emplacement. However, sub-vertical contacts

with the Bheinn Shuardail Dolostone are well exposed in quarries

GR(161925,819760), and advocate a boss-like morphology of the pluton, as proposed

by Harker (1904). More recent magnetic evidence produced by Hoersch (1979)

suggests that the granite is underlain by gabbro, refuting King’s theory of a sheet-like

emplacement King (1960). In addition to magnetic evidence, unrealistic rates of

erosion imply that the dolostone rafts are remenants of the pluton roof.

The notion that the Broadford anticline formed in response to forceful intrusion of

the Beinn an Dubhaich granite has been reviewed and discredited. If the anticline had

formed through forced intrusion of the granite, compressional deformation features

would be expected in the surrounding tertiary dykes, which have been inferred to

pre-date the granite. The concept of a passive emplacement is further substantiated

by a lack of enclaves within the granite, expected of a more forceful emplacement.

The ‘space problem’ associated with granite emplacement can, in this case, be solved

by doming of the crust, creating accommodation space for a passively intruding

granite within the Broadford anticline.

-57-

6 Geological History of Central Strath,

Skye

This chapter provides a summary of the history of Strath from the Pre-cambrian to

present day based on evidence from a study of the area and is illustrated by a series

of block diagrams.

The geological record of central Strath is initiated in the Pre-cambrian. At this time

north-west Britain was part of a large supercontinent, and was dominated by an arid

terrestrial environment. Fast-flowing braided river systems deposit the sandstones

of the Lonachan Sandstone Formation. A marked change in from the Lonachan

sandstone formation to the Bheinn Shuardail Dolostone formation represents an

unconformity in which sea level has elevated, inundating the Pre-cambrian bedrock

in a warm, clear, shallow, tropical marine (2).

The succeeding 200 years are not represented in the geological record of the Isle of

Skye as a consequence of powerful tectonic and erosive forces of the Caledonian

orogeny. Formidable compressive forces from the south-east has triggered the

Broadford thrust, responsible for the exhumation of the Lonachan Sandstone to the

surface (3-4). Stephenson and Merritt (2002) describe an overall marine regression

from shallow carbonate seas to arid continental planes in the Triassic period, which

marks the recommencing of the geological record. Subaerial exposure of these pre-

Caledonian formations is prerequisite for their deposition as intraformational clasts

within the Triassic Buidhe Conglomerate in an estuarine environment (5). The

succeeding Lower Jurassic sediments are characterised by a transgressive

depositional system with recurrent, cyclic fluctuations in sea level related to a

changing climate (6-7).

The intervening period between the end of Jurassic sedimentation and the initiation

of widespread igneous activity across central Strath is characterised by an episode of

compressional tectonics, fashioning the stratigraphic sequence of central Strath into

the large Broadford anticline and the adjoining Strathiad syncline (8).

-58-

Tertiary rifting of the North Atlantic resulted in increased amounts of igneous activity

across the UK, and consequently introduced a collection of diverse intrusive igneous

rocks to central Strath. The Strathiad dyke swarm has exploited a stress regime with

NE/SW directed minimum principal stress axes, creating a homogeneously NW/SE

orientated array of dykes. Doming of the earth’s crust in a proceeding compressive

tectonic episode provides space to host the Beinn an Dubhaich granite, which can be

seen cross-cutting the Strathiad dyke swarm (9). Intense heat from the Beinn an

Dubhaich granite has caused striking deformation of the Ordovician dolostones. A

series calc-silicate minerals, namely talc, tremolite, diopside, and forsterite, appear

sequentially within chert nodule reaction rims as the granite is approached as a

consequence of this phenomenon.

Tectonic uplift and tilting of the whole sequence is responsible for a gentle north-

easterly plunge of the Broadford anticline, and south-westerly plunge of the Strathiad

synform (10). The plunges of the folds are reflected in their present day outcrop

pattern, which has been generated by extensive post-Palaeogene erosion and

unroofing of the Beinn an Dubhaich granite (11).

-59-

Figure 55: A series of block diagrams illustrating each increment of the geological history of central

Strath.

-60-

7 Conclusion

To conclude this report, this chapter will summarise the main findings, and propose

some future work that would assist in revealing a more complete and

comprehensive geological history of central Strath.

The area of central Strath holds a diverse range of sedimentary rocks ranging in age

from Pre-cambrian to Early Jurassic. A collection of sedimentological and

palaeontological evidence allows the reconstruction of relative sea level throughout

the sedimentary record of central Strath. The changing sea level is summarised in

figure 56. The local stratigraphy has been intruded in to be a series of intrusive

igneous rocks of the British Tertiary Igneous Province. Orientations and structures of

Tertiary igneous rocks allows the estimation of an evolving stress regime through

time, and are likely to be a consequence of rifting of the NW Atlantic. Extensive

deformation of the rocks of central Strath manifests its self as a Caledonian thrust

fault, alongside a broad NE/SW trending antiform and its adjoining synform. The

antiform to the north of central Strath accommodates the Beinn an Dubhaich

Granite, which has metamorphosed the surrounding dolostones to form a series of

calc-silicate mineral assemblages around chert nodules.

Although many minerals within chert nodule reaction rims are visible to the naked

eye, thin section analysis would enhance this study by revealing microscopic minerals

within the reaction rims. Future work could include the collection of thin section

samples to establish more accurate isograds around the Beinn an Dubhaich granite,

enabling a more accurate cooling history to be inferred. More advanced methods

could be utilised to further our understanding of the emplacement of the Beinn an

Dubhaich granite, for example anisotropy of the magnetic susceptibility (AMS) study.

This would reveal any internal fabric of magnetic minerals within the granite, and

may give an indication of the mechanism that has emplaced the granite. A preferred

orientation of magnetic minerals would suggest a more forcible intrusion, thus may

have implications for the antiform that hosts the granite.

-61-

Figure 56: A summary of the marine transgressions and regressions that have been interpreted

throughout the formation of the stratigraphy of central Strath

-62-

8 References

Bailey, E. 1954. ‘Relations of Torridonian to Durness Limestone in the Broadford–

Strollamus district of Skye’. Geological Magazine, 91(01), 73.

Bédard, L. 1993. ‘Significance of Enclave Roundness: An Inherent Characteristic?’

The Journal of Geology, 101(1), 121-125.

Bell, B.R, Williamson, I.T. 2002. ‘Tertiary igneous activity. 371–407 in: The Geology

of Scotland’. Fourth edition. Trewin, N.H (editor). The Geological Society, London 1

Brück, P., Molyneux, S., Williams, M. and Woodcock, N. 2011. ‘A Revised Correlation

of Cambrian Rocks in the British Isles’. Geological Society, London, Special Report,

1(1), 1.

England, R. 1988. ‘The early Tertiary stress regime in NW Britain: evidence from the

patterns of volcanic activity’. Geological Society, London, Special Publications, 39(1),

381-389.

Farris, M., Oates, M. and Torrens, H. 1999. ‘New evidence on the origin and Jurassic

age of palaeokarst and limestone breccias, Loch Slapin, Isle of Skye’. Scottish Journal

of Geology, 35(1), 25-29.

Hallam, A. 2009. ‘Jurassic environments’ 1st ed. Cambridge [u.a.]: Cambridge

University Press, 132.

Hoersch, A. 1979. ‘General structure of the Skye Tertiary igneous complex and

detailed structure of the Beinn an Dubhaich Granite from magnetic evidence’

Scottish Journal of Geology, 15(3), 231-245.

Hoersch, A.L. 1981. ‘Progressive metamorphism of the chert-bearing Durness

Limestone in the Beinn an Dubhaich aureole, Isle of Skye, Scotland: a re-examination’

American Mineralogist, 66, 491–506.

Hesselbo, N. and Coe, A. 1989. ‘Jurassic sequence stratigraphy in the Hebrides

Basin, NW Scotland’. Marine and Petroleum Geology, 6(3), 243-260.

-63-

King, B. 1960. ‘The Form of the Beinn An Dubhaich Granite, Skye’. Geological

Magazine, 97(04), 326.

Miller, S. 2005. ‘Mountain geology’. The Edinburgh Geologist, (44), 4.

Moorbath, S. 1969, ‘Evidence for the age of deposition of the Torridonian sediments

of north-west Scotland’, Scottish Journal of Geology 5(2), 154-170.

Morton, N., Hesselbo, S., Oates, M. and Jenkyns, H. 1999. ‘The lower Lias Group of

the Hebrides Basin’ Scottish Journal of Geology, Vol. 34(1), 1998, pp. 23-60. Scottish

Journal of Geology, 35(1), 85-88.

Morton, N. and Hudson, J.D. 1995, ‘Field guide to the Jurassic of the Isles of Raasay

and Skye, Inner Hebrides, NW Scotland’. In: Field Geology of the British Jurassic (Ed.

by P.D. Taylor), The Geological Society, London, 209-280..

Morton, N. 1990, ‘Tectonic and eustatic controls of Jurassic genetic sequences in the

Hebrides Basin, SW Scotland’. Bulletin de la Societe Geologique de France, VI(6),

1001-1009.

Nicholas, C. 1994. ‘New stratigraphical constraints on the Durness Group of NW

Scotland’. Scottish Journal of Geology, 30(1), pp.73-85.

Owen, G. 1995, ‘Soft-sediment deformation in Upper Proterozoic Torridonian

sandstones (Applecross formation) at Torridon, northwest Scotland’, Journal of

Sedimentary Research 65A, 495-504.

Simms, M.J., Chidlaw, N., Morton, N. & Page, K.N. 2004, ‘British Lower Jurassic

Stratigraphy’, Geological Conservation Review Series, No. 30, Joint Nature

Conservation Committee, Peterborough, 324

Steel, R J and Wilson, A C, 1975. .Sedimentation and tectonism (?Permo-Triassic) on

the margin of the North Minch Basin, Lewis.. Journal of the Geological Society of

London, Vol.131, 183-202.

Stephenson, D. and Merritt, J. 2002, ‘Skye, A Landscape Fashioned by Geology’

Battelby: Scottish Natural Heritage, 2-5

-64-

Storetvedt, K.M. and Steel, R.J. 1977, ‘Palaeomagnetic evidence for the age of the

Stornoway formation’, Scottish Journal of Geology 13, 263-268.

Swett, K. 1969. ‘Interpretation of depositional and diagenetic history of Cambrian–

Ordovician succession of Northwest Scotland’. In: Kay, M. (ed.) North Atlantic –

Geology and Continental Drift, A Symposium. Memoir of the American Association of

Petroleum Geologists, 12, 630–646

Whitten, E. 1961, ‘Modal Variation and the Form of the Beinn an Dubhaich Granite,

Skye’, Geological Magazine, 98(06), 467.

Winter, J. 2001, ‘An introduction to igneous and metamorphic petrology’. 1st ed.

Upper Saddle River, NJ: Prentice Hall, 588-591.

-65-

9 Appendices

1) Field Slips 1-3

2) Field Notebooks 1-3

-66-

3) Dyke Analysis

-67-

4) Chert Nodule Analysis

-69-

5) Younging Table for Geological History

a geological report of central strath, skye

Science